Recent developments in internal combustion engine technology have simultaneously increased specific power, reduced fuel consumption, and reduced emissions. The performance improvements have been accomplished with the development of multi-valve engines, variable valve timing, direct fuel injection and application of forced induction (either supercharging, turbocharging, turbo-compounding, or a combination of supercharging and turbocharging).
Forced induction is used to increase engine power and efficiency. A forced induction engine is essentially two compressors in series. The compression stroke of the engine is the main compression that every engine has, and an additional compressor feeding into the intake of the engine makes it a forced induction engine. A compressor supplying a pressurized induction charge into the engine greatly increases the total compression ratio of the entire system, and this additional intake pressure is commonly referred to in the industry as boost.
A turbocharger relies on the volume and velocity of exhaust gases to spin (referred to herein synonymously as spool) a turbine wheel, which is connected to a compressor wheel via a common shaft. The boost pressure produced can be regulated by a system of release valves and electronic controllers. The chief benefit of a turbocharger is that it consumes less power from the engine than a supercharger; while the main drawback of a turbocharger is that engine response suffers greatly because it takes time for the turbocharger to come up to speed (referred to herein synonymously as spool up). The delay in power delivery from the turbocharger is referred to as turbo lag. Turbocharger design is inherently one of compromise in which a smaller turbocharger will spool quickly and deliver full boost pressure at low engine speeds, but boost efficiency will suffer at high engine revolutions per minute (RPM). It is appreciated that high and low values of RPM with respect to given engine are relative, with high RPM range typically being associated with acceleration of the vehicle. By way of example 3000 RPM is exemplary of a high RPM value for a conventional mid-size 6 cylinder sedan. A larger turbocharger, on the other hand, will provide improved high-RPM performance at the expense of lower RPM response. Other common design issues related to turbochargers include limited turbine lifespan, due to the high exhaust temperatures it must withstand, and the restrictive effect the turbine has upon exhaust flow. Superchargers, in contrast to turbochargers, have almost no lag time to build pressure because the compressor is always spinning proportionally to the engine speed. Superchargers are not as common as turbochargers because superchargers use the torque produced from the engine to operate; and, the torque utilized by the supercharger results in some loss in power and efficiency obtained from the engine.
An electric forced induction system utilizes an electric motor driven compressor to pressurize the intake air. By pressurizing the air available to the engine intake system, the air becomes denser, and is matched with more fuel, thereby producing increased horsepower. However, the power requirements and costs associated with electrically powering a compressor have limited commercial application of electric forced induction to-date.
Existing implementations of turbocharging, supercharging, and electric forced induction have met with growing acceptance but have limitations. For example, in turbocharging implementations, it is difficult to optimize compressor efficiency throughout the engine speed range, and that requires a compromise that accepts either lower efficiency and responsiveness at low RPMs to achieve higher efficiency at high rpm; or lower efficiency at high RPM to achieve better responsiveness and efficiency at low rpm. Turbocharging also suffers from turbo lag due to the inertial delay associated with accelerating the turbine wheels. The packaging of hot turbocharger components in the engine compartment is challenging, and there is heat transfer from the compressor to the intake charge under all operating conditions. There is also an undesirable back pressure associated with extracting exhaust heat energy with the turbocharger. In supercharging implementations, the power required to drive the supercharger—even when boost is not required (assumes a typical internal compressor by-pass) is a drain on the engine, as well as the power required to overcome supercharger inertia upon acceleration. There is unwanted heat transfer from the supercharger housing to the intake charge under all operating conditions, and additional packaging requirements for the supercharger. In addition, supercharging efficiency decreases at higher engine speeds and airflow. Electric forced induction implementations suffer from high power requirements and high costs associated with electrically powering a forced induction compressor and have limited commercial application to date.
While there have been many advances in forced induction, further improvements in combustion engine performance and efficiency are needed to meet mileage and performance requirements, while mitigating the problems and design limitations of existing implementations of forced induction. Thus, there exists a need for improved forced induction systems for improving combustion engine performance and efficiency.